Method of manufacturing semiconductor device

ABSTRACT

An impurity of a second conductivity type is selectively doped in a surface of a semiconductor substrate of a first conductivity type to form doped regions. A portion of a surface of the doped regions is covered by a heat insulating film. At least a remaining portion of the surface of the doped regions is covered by an absorbing film and the doped regions are heated through the absorbing film, enabling an impurity region of the second conductivity type to be formed having two or more of the doped regions that have a same impurity concentration and differing carrier concentrations.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.15/414,700, filed on Jan. 25, 2017, and allowed on Nov. 15, 2017, whichis based upon and claims the benefit of priority of the prior JapanesePatent Application No. 2016-053127, filed on Mar. 16, 2016, the entirecontents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

Embodiments of the present invention relate to a semiconductor devicethat uses silicon or a semiconductor material having a wide bandgap, anda method of manufacturing a semiconductor device.

2. Description of the Related Art

In semiconductor devices such as power devices, an edge terminationstructure called a junction termination extension (JTE) and that reducesthe level of charge stepwise in an outer peripheral direction from a p⁺region of an outer periphery of an active region in order to establishan element breakdown voltage level is often employed (for example, referto Published Japanese-Translation of PCT Application, Publication No.2001-508950).

Further, a second-conductivity-type region is selectively formed in afront surface or rear surface of a semiconductor substrate of a firstconductivity type and by changing the impurity concentration of thissecond-conductivity-type region, a semiconductor device is formed. In aJTE region, the carrier concentration is reduced stepwise on thesemiconductor substrate to control the level of charge.

FIG. 4 is a cross-sectional view of a traditional JTE structure. ASchottky barrier diode will be described as an example. For example, onan n⁺ semiconductor substrate 1 that is a material such as siliconcarbide (SiC), an n⁻ region 2 that is a drift layer is formed byepitaxial growth. On the n⁻ region 2, for example, an active portion p⁺region 3 implanted with, for example, aluminum (Al) is formed and forexample, a Schottky metal 5 such as titanium (Ti) is provided thereon.

In an outer periphery of the p⁺ region 3, a p region 41 having a lowcarrier concentration is formed and in a periphery farther outward, a p⁻region 42 having an even lower concentration is formed. With such aconfiguration, when voltage of a reverse direction is applied, electricfield concentration occurring in the edge termination structure isrelaxed, enabling a required breakdown voltage level to be established.

FIG. 5 is a cross-sectional view of a traditional space-modulated JTEstructure. The ratio of the p⁻ region 42 to the p region 41 variesstepwise with distribution whereby the extent of electric fieldconcentration may be further relaxed and the breakdown voltage level maybe further improved. Such an edge termination structure is a technologyused not only in Schottky barrier diodes but also in MOS power devicessuch as metal oxide semiconductor field effect transistors (MOSFET),insulated gate bipolar transistors (IGBT), and the like.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, semiconductor deviceincludes an impurity region of a second conductivity type, selectivelyprovided in a surface of a semiconductor substrate of a firstconductivity type. The impurity region of the second conductivity typehas two or more regions that have a same impurity concentration anddiffering carrier concentrations.

In the semiconductor device, the two or more regions have an impurityactivation rate that differs according to region.

The semiconductor device includes an active region and an edge regionsurrounding the active region, as semiconductor device structure. Thetwo or more regions are provided as an edge termination structure of theedge region.

In the semiconductor device, the semiconductor substrate is formed ofone of silicon carbide, gallium nitride, silicon, and diamond.

According to another aspect of the present invention, a method ofmanufacturing a semiconductor device in which an impurity region of asecond conductivity type is selectively formed in a surface of asemiconductor substrate of a first conductivity type, includesselectively doping an impurity of the second conductivity type in thesurface of the semiconductor substrate and forming a doped region;covering a portion of a surface of the doped region with a heatinsulating film; covering at least a remaining portion of the surface ofthe doped region with an absorbing film; and heat treating the dopedregion through the absorbing film.

In the method of manufacturing a semiconductor device, a film thatabsorbs 10% or more of energy of light is used as the absorbing film.

In the method of manufacturing a semiconductor device, carbon is used asthe absorbing film.

In the method of manufacturing a semiconductor device, the heat treatingis performed by optical heating.

In the method of manufacturing a semiconductor device, the heat treatinguses, as a light source, one of a halogen lamp, an excimer lamp, a YAGlaser, an excimer laser, a CO₂ laser, and a semiconductor laser.

In the method of manufacturing a semiconductor device, a material havinga heat conduction rate of 50 W/m·K or less is used as the heatinsulating film.

In the method of manufacturing a semiconductor device, a materialcontaining silicon nitride or silicon oxide is used as the heatinsulating film.

Objects, features, and advantages of the present invention arespecifically set forth in or will become apparent from the followingdetailed description of the invention when read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view for describing heat treatment at a timeof formation of a typical JTE structure;

FIG. 2 is a cross-sectional view depicting a state during formation of aJTE structure according to a first embodiment of the present invention;

FIG. 3 is a cross-sectional view depicting a state during formation of aJTE structure according to a second embodiment of the present invention;

FIG. 4 is a cross-sectional view of a traditional JTE structure; and

FIG. 5 is a cross-sectional view of a traditional space-modulated JTEstructure.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described in detail withreference to the accompanying drawings. In the present description andaccompanying drawings, layers and regions prefixed with n or p mean thatmajority carriers are electrons or holes. Additionally, + or − appendedto n or p means that the impurity concentration is higher or lower,respectively, than layers and regions without + or −. In the descriptionof the embodiments below and the accompanying drawings, main portionsthat are identical will be given the same reference numerals and willnot be repeatedly described. Further, in the present description, whenMiller indices are described, “−” means a bar added to an indeximmediately after the “−”, and a negative index is expressed byprefixing “−” to the index.

First, a method of forming a JTE structure in a typical Schottky barrierdiode will be described.

In a typical Schottky barrier diode (refer to FIG. 4), when thesemiconductor substrate 1 is formed of a material such as SiC, animpurity such as implanted Al, etc. does not become a carrier as it isand the impurity first becomes a carrier by being taken into thematerial structure of the semiconductor substrate 1 consequent to heattreatment at 1000 degrees C. or higher. This process is calledactivation.

FIG. 1 is a cross-sectional view for describing the heat treatment atthe time of formation of a typical JTE structure. As depicted in FIG. 1,in the p⁺ region 3 of the active portion, and in the p region 41 and thep region 42 of the JTE, an impurity of respectively differingconcentrations is implanted. Thereafter, a substrate stage 6 is heatedby high-frequency induction heating, etc., and the semiconductorsubstrate 1 is heated by radiation or heat conduction from the substratestage 6 whereby the p⁺ region 3 of the active portion and the p region41 and p⁻ region 42 of the JTE are activated.

Here, a substrate surface (main surface) of the semiconductor substrate1 is covered by a cap film 81 in advance so as to not become rough fromheat treatment and the cap film 81 is removed after the heat treatment.

FIG. 2 is a cross-sectional view depicting a state during formation of aJTE structure according to a first embodiment of the present invention.In the first embodiment, as depicted in FIG. 2, an impurity of Al, etc.is implanted in the p regions 41, 42 (although described hereinafter,the region 42 becomes a p⁻ region) of the JTE and, in the p⁺ region 3 ofthe active portion to be formed selectively on the semiconductorsubstrate 1 made from a material of a first conductor. At this time, theregions 41 and 42 have a common impurity concentration. Thesemiconductor substrate 1 uses one of silicon carbide, gallium nitride,silicon, and diamond.

Thereafter, on a portion of the p region 42 on the semiconductorsubstrate 1, a heat insulating film 7 having a low heat conduction rateand a heat insulating property, in particular, a heat insulating filmformed of, for example, silicon nitride, silicon oxide, etc. having aheat conduction rate of 50 W/m·K or less, is formed.

Next, an absorbing film 82 of, for example, carbon, etc. and absorbinglight energy and generating heat is formed so as to cover the region 3,the region 41, and the region 42 (a portion formed by the p⁺ region 3,the p region 41, and the p region 42 formed of a material of a secondconductor). Here, when heat conduction from a substrate stage is used aslater-stage heating, a function of absorbing light energy need not berequired of the absorbing film 82 and a material having good heatconduction suffices.

The absorbing film 82 is heated and the implanted impurity is activatedby a heat flux from the absorbing film 82. In particular, the absorbingfilm 82, for example, is irradiated with heating light 9 using a lightsource such as a halogen lamp, an excimer lamp, etc., or a laser lightsource such as YAG, an excimer, CO₂, a semiconductor, etc. The absorbingfilm 82 absorbs the energy of the heating light 9 and generates heat.Here, a method of generating heat may be any method provided theabsorbing film 82 is heated and, for example, radiation or heatconduction from the substrate stage may be used. Here, when heating isby heat conduction, to ensure uniformity in the surface, the substratestage and the absorbing film 82 have to be uniformly adhered. Incontrast, when heating light radiation, etc. is used, an advantage isthat irrespective of surface formation of the substrate, uniformity maybe ensured.

When heating light is used, a thickness and an absorption coefficient ofthe material of the absorbing film 82 are set so that the absorbing film82 provides a sufficient heating temperature to the region 3, the region41, and the region 42 by the light energy of the heating light 9 (sothat at least 10% or more of the light energy of the heating light 9 maybe absorbed by the portion including the region 3, the region 41, theregion 42).

The region 3 and the region 41 are heated by a heat flux 8 h 1 from theabsorbing film 82 and the region 3 and the region 41 are activatedsimilar to a traditional case whereby respective p⁺ and p regions (thep⁺ region 3 and the p region 41) are formed.

On the other hand, in the region 42, since a heat flux 8 h 2 smallerthan the heat flux 8 h 1 is provided by the heat insulating film 7, theheating temperature is lower than in the region 41. The rate at whichthe implanted impurity is activated to carriers is dependent on theheating temperature and at a low heating temperature, the activationrate is low whereby the region 42 (the p⁻ region 42) has a lower carrierconcentration than the region 41.

Accordingly, by optimizing the material, thickness, irradiationconditions of the heating light, etc. of the heat insulating film 7, acarrier concentration corresponding to the p⁻ region 42 of the JTEstructure may be imparted. The heat insulating film 7 may preferablyhave a film heat conduction rate of 50 W/m·K or less, and siliconnitride, silicon oxide, etc. may be applied. For example, on top ofsetting the thickness of the heat insulating film 7 to be about 0.3 μm,it suffices to irradiate the heating light under a condition that theregion 41 reaches about 1700 degrees C.

When an impurity concentration of the regions 41 and 42 is set to be2.5×10¹³ cm⁻², and an activation rate of the region 42 is set to be 20%against an activation rate of 40% for the region 41, the carrierconcentration is 1.0×10¹³ cm⁻² in the region 41, and is 0.5×10¹³ cm⁻² inthe region 42.

FIG. 3 is a cross-sectional view depicting a state during formation of aJTE structure according to a second embodiment of the present invention.In the second embodiment, an example of a JTE structure in whichcompared to the first embodiment, the extent of electric fieldconcentration is further relaxed and the breakdown voltage level israised will be described.

As depicted in FIG. 3, the second embodiment is an example of a JTEconfigured by the 3 regions of the p region 41, the p⁻ region 42, and ap⁻⁻ region 43.

In the second embodiment, on the semiconductor substrate 1, the heatinsulating film 71 and a heat insulating film 72 are combined andformed, and the heat insulating film thickness of the heat insulatingfilm 71 of a region 42 portion and the heat insulating film thickness ofthe heat insulating film 72 of a region 43 portion are changed. In theconfiguration example in FIG. 3, on the regions 42 and 43, the heatinsulating film 71 is formed, and on the heat insulating film 71 in theregion 43 portion, the heat insulating film 72 is further formed.

The thicknesses of the heat insulating films 71 and 72 of the regions 42and 43, for example, are 0.3 μm and 0.5 μm, respectively. As a result,in the region 43, consequent to a higher insulating effect, a heatingtemperature that is one step lower results whereby a carrierconcentration corresponding to the p⁻⁻ region is obtained.

In each of the described embodiments, an example through a heatinsulating film between an absorbing film and an impurity region hasbeen described. Without limitation to formation of such a heatinsulating film, for each region (for example, regions 41, 42, 43),direct variance of the irradiation period and the intensity of theheating light irradiated on the absorbing film 82 may obtain the sameworking effect.

For example, when laser light is swept in the absorbing film 82 surfaceby a laser light source to perform heating, by changing the sweepingspeed, sweeping frequency according to region, the heating temperatureof each region may be controlled.

As described, according to the embodiments of the present invention, thenumber of processes of impurity implantation for forming a JTE regionmay be reduced, enabling a problem of increased product cost thattraditionally occurs to be solved.

In the description above, although formation of an edge terminationstructure of a Schottky barrier diode has been described, withoutlimitation hereto, application is similarly possible in a case of avertical MOS-type structure having an active region that is a MOSFET,IGBT, etc. and with respect to a rear structure MOSFET, IGBT, etc.

In addition, application is possible in a structure in which a verticalMOSFET and a vertical Schottky barrier diode are integrated together ona single chip and the carrier concentration on a rear surface side ofthe semiconductor substrate is to be partially changed. Moreover,application is possible in the formation of a rear side region of aRC-IGBT in which an IGBT and a diode are integrated together.

In the embodiments above, although a case of a SiC semiconductorsubstrate has been described as an example, without limitation hereto,realization is possible with a semiconductor substrate of silicon, GaN,etc.

In configuring the traditional JTE structure above, implantation of animpurity has to be repeated for each region of a different carrierconcentration. Moreover, to obtain an optimal concentration distributionin a depth direction in materials in which thermal diffusion of animpurity is difficult such as SiC, even within one region, the energyhas to be changed while repeatedly performing implantation work manytimes.

Therefore, the number of man-hours required in the implantationincreases or overload of the impurity implantation apparatus occurswhereby problems of reduced operating ratio of the manufactured deviceoverall and increased product cost occur.

However, according to the invention described above, to form regions ofdiffering carrier concentrations on the semiconductor substrate withouthaving to repeatedly perform impurity implantation of differingconcentrations for each region, impurity implantation is performed by acommon concentration whereby regions of differing carrier concentrationsmay be formed by making the heating temperature for activating eachregion different to control the activation rate. As a result, impurityimplantation which has a large man-hour burden may be reduced, enablingsimple manufacturing without increasing the product cost.

According to the present invention, an effect is achieved in thatregions of differing carrier concentrations may be easily formed on asemiconductor substrate.

As described, the present invention is suitable for semiconductorelements that use silicon, a wide bandgap, etc. as a semiconductormaterial and, for example, is useful in inverters, switching powersources, etc.

Although the invention has been described with respect to a specificembodiment for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

What is claimed is:
 1. A method of manufacturing a semiconductor devicein which an impurity region of a second conductivity type is selectivelyformed in a surface of a semiconductor substrate of a first conductivitytype, the method comprising: forming a doped region in the surface ofthe semiconductor substrate by selectively doping an impurity of thesecond conductivity type in the surface of the semiconductor substrate;covering a portion of a surface of the doped region with a heatinsulating film; covering at least a remaining portion of the surface ofthe doped region with an absorbing film; and heat treating the dopedregion through the absorbing film.
 2. The method of manufacturing asemiconductor device according to claim 1, wherein the absorbing film isa film that absorbs 10% or more of energy of light.
 3. The method ofmanufacturing a semiconductor device according to claim 1, wherein theabsorbing film is carbon.
 4. The method of manufacturing a semiconductordevice according to claim 1, wherein the heat treating is performed byoptical heating.
 5. The method of manufacturing a semiconductor deviceaccording to claim 1, wherein the heat treating uses, as a light source,one of a halogen lamp, an excimer lamp, a YAG laser, an excimer laser, aCO₂ laser, and a semiconductor laser.
 6. The method of manufacturing asemiconductor device according to claim 1, wherein a material having aheat conduction rate of 50 W/m·K or less is used as the heat insulatingfilm.
 7. The method of manufacturing a semiconductor device according toclaim 1, wherein a material containing silicon nitride or silicon oxideis used as the heat insulating film.
 8. The method of manufacturing asemiconductor device according to claim 1, wherein the doped region isheat treated to form two or more regions that have a same impurityconcentration and differing carrier concentrations.
 9. The method ofmanufacturing a semiconductor device according to claim 1, whereincovering at least the remaining portion of the surface of the dopedregion with an absorbing film includes covering the heat insulating filmwith the absorbing film, and wherein heat treating the doped regionthrough the absorbing film includes heat treating a portion of the dopedregion covered by the heat insulating film and heat treating a portionof the doped region not covered by the heat insulating film.